This invention relates generally to methods and apparatus for electrostatographic recording using multi-bit rendering devices.
In general, in electrostatographic recording it is known to form electrostatic latent images on a xerographic surface by first uniformly charging a charge retentive surface such as a photoreceptor. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
In U.S. Pat. Nos. 5,258,849 and 5,258,850, there is taught that higher image quality can be achieved using multi-bit rendering. In multi-bit rendering, various shades of a color or gray may be achieved by selectively imagewise exposing the photoreceptor at different minute areas or pixel recording areas to different amounts of exposure so that different amounts of toner are selectively attracted to such areas. The results are that the pixel areas develop different densities according to such exposures and improved tone scale rendition is possible.
Improvements in the art of electrostatographic recording include the capability of recording in two or more colors. One aspect of this technology which has developed some attention in multiple color recording is tri-state highlight xerography which has evolved primarily as tri-level highlight xerography. The term tri-state highlight xerography as used herein implies that three states may exist on the same image flame allowing development of certain areas in one color, certain other areas in another color and still other areas with either a third color or no development.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929. This patent discloses the use of tri-level, xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In U.S. Pat. No. 5,347,345, there is disclosed an improvement upon tri-level color xerography wherein two-color images in a single pass is provided but using toners of the same polarity. This provides an image that is more readily transferred to a receiver sheet.
The interest in tri-level xerography appears directed to several factors. Firstly, registration of the color images can be better controlled since the separate color latent images are recorded by the same exposure source on the same image frame at the same time. Thus, registration of the separate latent images can be made down to the pixel resolution of the recording source. A second factor involves productivity since only one image frame is used to record a two-color developed image and only one transfer of the image to a receiver sheet is required, hence, the term xe2x80x9csingle-passxe2x80x9d is associated with this technology.
In U.S. Pat. No. 5,241,359, there is disclosed an image creation apparatus operable in a tri-level highlight color imaging or a black monochrome mode. The developer structures are biased in the tri-level mode using a chopped DC bias while in the monochrome black mode only the black developer structure is biased using a standard monochrome bias.
In U.S. Pat. No. 5,305,069, a single-pass, two-color tri-level xerographic apparatus is described wherein improved tone rendition is provided using a dithering approach wherein several tone scales are provided by combinations of several adjacent pixels. Thus, an area of 4xc3x974 pixels, say red, may appear more dense than an area of 2xc3x972 red pixels and so on. In order to accomplish imaging for the two colors, a current driver to a laser is supplied supplemental current when recording in the red color whereas for recording the white background, no supplemental current is provided. In addition to the dithering method, this patent discloses the creation of density tone gradation by laser beam irradiation per pixel wherein the irradiation period is controlled by a pulsewidth modulation in accordance with image density signals.
A problem with the prior art is that the improved image quality obtainable using multi-bit writers has not been effectively utilized in copiers/printers that are able to form color images in a single-pass, as well as a single-color image. The invention and its objects address this problem.
In accordance with the invention, there is provided a method and apparatus of forming images on a charge retentive surface using a single writer source, the invention including uniformly charging said surface; exposing in one image frame of said surface on a pixel by pixel basis with varying amounts of exposure by said source for at least some pixels to form a latent image including a) a charged area developable (CAD) latent image having at least some exposed pixels at various different sub-levels of exposure within said charged area developable image, (b) a discharged-area developable (DAD) latent image having at least some exposed pixels at various different sub-levels of exposure within said discharged area developable image, and (c) a background area; developing said DAD and CAD latent images with toners of two different colors from respective two different developer sources; exposing a second image frame of said uniformly charged surface on a pixel by pixel basis with varying amounts of exposure by said source for at least some pixels to form a monochrome latent image wherein at least some exposed pixels are exposed at various different levels of exposure; and developing said monochrome latent image using one of said developer sources and wherein a voltage bias on said one of said developer sources is adjusted to a different bias when operating in a monochrome exposure mode than when operating in a two-color exposure mode.
In accordance with another aspect of the invention, there is provided a method and apparatus of forming two-color images on a charge retentive surface using a single writer source, the invention including uniformly charging said surface; exposing in one image frame of said surface on a pixel by pixel basis with varying amounts of exposure by said source at least some pixels to form a latent image including (a) a charged area developable (CAD) latent image having at least some pixels within said charged area developable image, (b) a discharged-area developable (DAD) latent image having at least some pixels within said discharged area developable image, and (c) a background area; developing said CAD and DAD latent images with toners of two different colors from respective two different developer sources; and including the step of processing signals representing a pixel having multicolor components in accordance with a set of criteria to determine whether the pixel is to be recorded on said one image frame as one color or the other color of said two different colors.
In accordance with yet another aspect of the invention, there is provided a method of forming images on a charge retentive surface using a single writer source, said method comprising the steps of uniformly charging said surface; exposing in one image frame of said surface on a pixel by pixel basis with varying amounts of exposure by said source for at least some pixels to form a latent image including (a) a charged area developable (CAD) latent image having at least some exposed pixels at various different sub-levels of exposure within said charged area developable image, (b) a discharged-area developable (DAD) latent image having at least some exposed pixels at various different sub-levels of exposure within said discharged area developable image, and (c) a background area; developing said DAD and CAD latent images with toners of two different colors from respective two different developer sources; exposing a second image frame of said uniformly charged surface on a pixel by pixel basis with varying amounts of exposure by said source for at least some pixels to form a monochrome latent image wherein at least some exposed pixels are exposed at various different levels of exposure; developing said monochrome latent image using one of said developer sources and wherein the different levels of exposure in the monochrome latent image mode and the different sub-levels of exposure in at least one of the CAD and DAD latent images have the exposures uniformly stepped in lightness space.
In accordance with still another aspect of the invention, there is provided a method of forming two-color images on a charge retentive surface using a single writer source, said method comprising the steps of uniformly charging said surface; exposing in one image frame of said surface on a pixel by pixel basis with varying amounts of exposure by said source at least some pixels to form a latent image including (a) a charged area developable (CAD) latent image having at least some pixels within said charged area developable image, (b) a discharged-area developable (DAD) latent image having at least some pixels within said discharged area developable image, and (c) a background area; developing said CAD and DAD latent images with toners of two different colors from respective two different developer sources; and including the steps of modifying a value associated with a pixel with an error value associated with rendering of prior processed pixels to obtain a modified pixel value and in response to signals representing said modified pixel value determining whether the pixel is to be recorded on said one image frame as one color or the other color of said two different colors.